Coaxial line to microwave cavity coupling section comprising a waveguide beyond cutoff

ABSTRACT

A variable microwave coupler between a coaxial line and an EPR microwave cavity. A housing includes the resonant cavity as well as a section dimensioned to be a waveguide. Energy from the coaxial line is coupled into this section. The section is dimensioned to be a waveguide beyond cutoff and it has an opening into the resonant cavity. A planar loop located in the section is connected to be excited by the energy in the line. The loop is rigidly positioned so that its plane is substantially at right angles to a microwave magnetic field in the cavity. A microwave energy radiator is positioned in the section to be inductively coupled with the microwave energy from the loop. The radiator is positioned in the section, adjacent an intersection between the section and cavity, to electromagnetically radiate the energy inductively coupled to it into the cavity. Inductive coupling between the loop and the radiator is selectively controlled to vary the amount of energy electromagnetically radiated by the radiator into the cavity.

United States Patent [191 Hyde [ COAXIAL LINE T0 MICROWAVE CAVITYCOUPLING SECTION COMPRISING A WAVEGUIDE BEYOND CUTOFF [75] lnventor:James Stewart Hyde, Menlo Park,

Calif.

[73] Assignee: Varlan Associates, Palo Alto, Calif.

[22] Filed: Feb. 28, 1974 [21] App]. No.: 446,843

Related [1.8. Application Data [62] Division of Ser. No. 370,177, JuneI4, 1973, Pat. No.

[4 1 July 22,1975

Primary Examiner- Paul L. Gensler Attorney, Agent, or Firm-Stanley 2.Cole; Gerald M. Fisher [57] ABSTRACT A variable microwave couplerbetween a coaxial line and an EPR microwave cavity. A housing includesthe resonant cavity as well as a section dimensioned to be a waveguide.Energy from the coaxial line is coupled into this section. The sectionis dimensioned to be a waveguide beyond cutoff and it has an openinginto the resonant cavity. A planar loop located in the section isconnected to be excited by the energy in the line. The loop is rigidlypositioned so that its plane is 52 us. Cl. 333/21 R; 333/24 R; 333/98 RSubstantially right angles to a microwave magnetic [51] im. Cl H0lp5/04; l-lOlp 5/08 i l In P cavltymlcrowaye e gy radiator IS 9 5 Field fSearch 333/21 R 33, 83 R, 9 R. sitioned in the section to be inductivelycoupled with 333/24 R themwrowave energy from the loop. The rad ator ispositioned in the section, ad acent an intersection be- 5 ReferencesCited tween the section and cavity, to electromagnetically UNITED STATESPATENTS rad ate the energy inductively coupled to it into the 2 973 4912/l961 Maciszewski 333/24 R sii lvduclnve- (fuplmg if z i the and ra laor [8 se ective y contro e o vary t e amount 0 3,423,70l 1/l969 Kach333/83 R gy electromagneticany radiated y the radiator into the cavity.

12 Claims, 3 Drawing Figures T0 INDICATOR 5 i i I? 3 l 26 n l BRIDGE 37|s Ih 22 'b I X l 0 X 1 3 38 3 n 55 l4 WAVEGUIDE BEYOND 27 I8 CUTOFF 392 COAXIAL LINE T MICROWAVE CAVITY COUPLING SECTION COMPRISING AWAVEGUIDE BEYOND CUTOFF This is a division of application Ser. No.370,177, filed June 14, 1973, now U.S. Pat. No. 3,828,244.

FIELD OF THE INVENTION The present invention relates generally tomicrowave couplers between coaxial lines and microwave cavities and,more particularly, to a coupler wherein an electromagnetic microwaveradiator for the cavity is inductively coupled to a termination of thecoaxial line.

BACKGROUND OF THE INVENTION Variable couplers between coaxial lines andmicrowave cavities, such as resonant cavities or waveguides, typicallyemploy rotating loops as a termination for the coaxial line. Therotating loop varies the polarization angle of microwave energy coupledbetween the coaxial line and microwave cavity, thereby varying thedegree of coupling between the line and cavity. U.S. Pat. No. 3,2 l 4,684 assigned to the present assignee is exemplary of such rotating loopcouplers. Rotating loops require use of a sliding contact between therotating element and the coaxial line. Such contacts introduce noise andattenuation in the coupled energy and are relatively susceptible to opencircuiting.

SUMMARY OF THE INVENTION In accordance with the present invention thereis provided a new and improved coaxial line to microwave cavity variablecoupler that does not require movable elements that are electricallyconnected to any portion of the coaxial line, thereby obviating the needfor brushes. The result is achieved by providing the cavity with asection that is electrically coupled to a termination of the coaxialline. The termination is rigidly connected in situ between the coaxialline center conductor and shield and functions as an inductive radiatorfor the electromagnetic energy. The section is dimensioned so that it iseffectively a waveguide having a cutoff frequency beyond the frequencypropagating in the coaxial line, whereby electromagnetic wavetransmission is strongly attenuated in the section. Positioned in thesection and inductively coupled to the coaxial line termination is anelectromagnetic wave radiator that excites the cavity with the microwaveenergy induced therein from the coaxial line termination. Variations inthe inductive coupling between the radiator and coaxial line terminationare provided by adjusting the position of the radiator in the waveguidebeyond cutoff sec tion to control the coupling between the coaxial lineand cavity. Variable coupling can be provided by forming the radiator asa metal stud that is inserted to differing extents in the section or byforming the radiator as a rotatable disc that variably interceptsdifferent amounts of energy from the coaxial line termination.

In a preferred embodiment the coaxial line termination is a planar loopextending between the coaxial line center conductor and outer conductor.The loop, in combination with the radiator, excites the cavity so that amagnetic field subsists in the cavity at right angles to the plane ofthe loop.

It is accordingly an object of the present invention to provide a newand improved coupler between a microwave cavity and a coaxial line.

Another object of the present invention is to provide a variable,relatively noise-free coupler between a coaxial line and a microwavecavity, which coupler is not susceptible to open circuits between amoving element and the coaxial line.

Another object of the invention is to provide a new and improvedvariable coupler between a coaxial line and a microwave cavity, whichcoupler does not require any moving parts to be connected to the coaxialcable.

Still another object of the invention is to provide a new and improvedvariable coupler between a coaxial line and a microwave cavity, whichcoupler does not employ sliding contacts.

A further object of the invention is to provide a microwave couplerbetween a coaxial line and a microwave cavity wherein energy isinductively coupled between a tennination of the coaxial line and aradiator into the cavity.

An additional object of the invention is to provide a new and improvedmicrowave coupler between a coaxial line and an EPR spectrometerresonant cavity.

The above and still further objects, features and advantages of thepresent invention will become apparent upon consideration of thefollowing detailed description of several specific embodiments thereof,especially when taken in conjunction with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a side sectional view of onepreferred embodiment of the invention;

FIG. 2 is a side sectional view taken along the line 2-2 of FIG. 1; and

FIG. 3 is a side sectional view of another embodiment of the invention.

DETAILED DESCRIPTION OF THE DRAWING Reference is now made specificallyto FIGS. 1 and 2 of the drawing wherein there is illustrated a preferredembodiment of the variable coupler of the present invention for feedingmicrowave energy of oscillator 11 to a microwave cavity 12 via bridge 10and coaxial line 13. In the preferred embodiment, cavity 12 includes arectangular resonant cavity section 14 formed as a right parallelepipedin which there are excited oppositely polarized and orthogonal magneticand electric field vectors. The oppositely polarized magnetic fieldvectors are indicated by the crosses 15 and dots 16 that are directed,in the cross-sectional view of FIG. 1, in planes parallel to the upperand lower walls 17 and 18 of resonant cavity 14. The oppositelypolarized electric field vectors l9 and 20 extend at right angles to andbetween walls 17 and 18, with a null electric field vector beingprovided substantially along a medium plane 22 of the resonant cavity14. The microwave electric and magnetic field vectors l5, l6, l9 and 20thereby excite resonant cavity 14 in the rectangular TE mode. Positionedmidway between wall 17 and 18 on medial plane 22 is a hollow dielectricsample holder 23 which is preferably formed as a capillary tube for aliquid sample to be analyzed by EPR spectroscopy techniques. It is to beunderstood that resonant cavity 14 can take other geometricalconfigurations (e.g., cylindrical), that the resonant cavity can beexcited to other modes, and that sample holder 23 can have otherconfigurations and positions in the cavity.

To enable the EPR spectroscopy analysis of the sample in holder 23 to beperformed, a modulation field is coupled to the sample, with themagnetic field lines of flux extending in planes parallel to andincluding the electric field vectors 19 and 20. A strong polarizingmagnet including oppositely polarized pole faces 24 and 25 is provided,with the pole faces being disposed parallel to walls 17 and 18 ofresonant cavity 14. in a gap between pole faces 24 and 25 and cavitywalls 17 and 18, coils 26 and 27 are respectively disposed. Coils 26 and27 are driven by a relatively low frequency AC source 28, typicallyhaving a frequency on the order of lOOKl-lz to modulate the DC magneticfield provided by pole faces 24 and 25. To provide an indication of thespectral lines of the sample in holder 23, bridge is provided to coupleenergy from microwave source 11 to coaxial line 13 and to be responsiveto energy reflected from resonant cavity 14. Bridge 29 includes detectorcircuitry which drives an indicator in a manner well known to thoseskilled in the art.

The structure of FIGS. 1 and 2 described to the present is well known tothose skilled in the art and does not form a particular part of thepresent invention. The present invention is concerned with an improveddevice for variably coupling microwave energy from coaxial line 13 toresonant cavity 14.

The improved variable coupler preferably includes a section 31 formed asa cylindrical section at one end of cavity 12. Alternatively, section 31could be other shapes, such as a parallelepiped. Section 31 is, inessence, a relatively short length of waveguide dimensioned so that thewaveguide cutoff frequency is greater than the frequency of source 11that drives coaxial line 13 and the resonant frequency of resonantcavity 14. Various manuals provide the formula for calculating thecutoff frequency for waveguides, i.e., for cylindrical waveguides seeMicrowave Engineers Handbook, Horizon House, 1971, Vol. I, p 34.Accordingly, microwave energy of source 11 coupled into section 31cannot be supported in section 31 for electromagnetic wave propagationand is rapidly attenuated in section 3].

Microwave energy from source 11 is coupled into section 31 by providingcoaxial cable 13 with a termination in the form of fixedly mountedplanar loop 34 that is connected between center conductor 35 and shield36 of coaxial line 13. The connection to shield 36 is provided by shortcircuiting one end of loop 34 to terminating wall 37 of section 13 andby short circuiting shield 36 to the exterior of metal cavity 12. Loop34 is rigidly positioned in section 31 so that the loop lies in a planethat is substantially at right angles to walls 17 and 18 as well asmicrowave magnetic field vectors and 16. In response to the microwavecurrent flowing in loop 34 there is a microwave magnetic field inducedin section 31 at right angles to the plane of the loop and in the sameplane as the microwave magnetic field vectors 15 and 16 in resonantcavity 14. Attenuation of section 31 for the energy of source 11 is suchthat with screw 39 withdrawn transmission line 35 is a few dbundercoupled to resonator 14. Insertion of a sample holder 23 withsamples having dielectric loss causes the resonator to become moreovercouple' and subsequent insertion of screw 39 permits matching of thetransmission line 35 to the resonator 14.

The magnetic field derived from loop 34 is coupled to metallic stud 38that is positioned in section 31 close to the mouth of the section wherethe section intersects resonant cavity 14. Metal stud 38 functionsbasically as a dipole radiator for the microwave energy toelectromagnetically excite resonant cavity 14 at the same frequency asthe energy induced in the radiator from loop 34. Stud 38 is preferablyfabricated of a non-ferromagnetic metal, such as silver or aluminum, sothat it does not distort the magnetic lines of flux coupled to theresonant cavity by magnet pole faces 24 and 25 in combination with coils26 and 27. Stud 38 is positioned in section 31 so that its longitudinalaxis is coplanar with the plane of loop 34 and generally perpendicularwith the microwave magnetic field vectors 19 and 20.

To control the amount of microwave energy coupled into resonant cavity14, the length of stud 38 in section 31 is adjustable. To this end, stud38 is mounted on the end of a dielectric screw 39 which is movable in athreaded bore of cavity 12, which threaded bore extends at right anglesto and through cavity wall 18. in response to screw 39 being turned todrive stud 38 so that it is inserted to a greater extent into section31, there is a greater amount of coupling from loop 34 to stud 38. inresponse to the coupling in stud 38, the stud functions as a dipoleradiator to excite cavity 14 to provide the microwave electric andmagnetic field vectors 15, 16, 19 and 20. ln response to screw 39 beingrotated so that stud 38 is withdrawn from section 31, the inducedcurrent in stud 38 decreases and the intensity of the microwave fieldsin cavity 14 is accordingly reduced. Thereby, variable insertion of stud38 in section 31 controls the amount of microwave energy coupled fromcoaxial line 13 to resonant cavity 14 without having any physicalconnection to the coaxial line, all parts and connections to which arerigidly connected in situ at all times.

In accordance with a modification of the invention, as illustrated inFIG. 3, inductive coupling between loop 34 and the dipole radiator forcavity 14 is obtained by providing a rotatable non-ferrous metal disc 41that is rotatable about an axis that is at right angles to the plane ofloop 34. To this end, one end of disc 41 is rigidly connected to one endof dielectric screw 42 that is coaxial with the disc axis of rotation.Screw 42 fits in a threaded bore extending through cavity wall 43 thatis disposed at right angles to cavity walls 17 and 18. In response torotation of screw 42, disc 41 is correspondingly rotated and translatedto vary the amount of mag netic coupling between the disc and loop 34and thereby vary the degree of coupling between coaxial line 13 andresonant cavity 14.

While there have been described and illustrated several specificembodiments of the invention, it will be clear that variations in thedetails of the embodiments specifically illustrated and described may bemade without departing from the true spirit and scope of the inventionas defined in the appended claims. For example. the principles of theinvention are not necessarily limited to use in connection with EPRspectrometer resonant cavities and the coupler can be employed betweenany coaxial line and any microwave cavity capable of electromagneticallysupporting the frequency in the line. Thereby, the term microwave cavityis to be c. nstrued to cover waveguides as well as resonant cavities.

What is claimed is:

1. A variable coupler between a coaxial line and a microwave cavity forsupporting electromagnetic energy at a frequency propagating in saidline comprising a section connected to and opening into said cavity,said section being dimensioned so that it is a waveguide beyond cutofffor the frequency propagating in said line, a fixedly mounted planarloop in said section connected to be in energy exchange relationshipwith the energy propagating in said line and rigidly positioned so thatits plane is substantially at right angles to a microwave magnetic fieldcoupled into the cavity, an electro magnetic radiator for the energy,said radiator being positioned in the section, said radiator being onlyinductively coupled with said loop, said radiator being positioned inthe section in electromagnetic energy exchange relationship with thecavity so that there is coupling between the cavity and the loop via theradiator of the energy inductively coupled between the loop andradiator, and means for controlling the coupling between the loop andthe radiator to control the coupling between the cavity and line.

2. The coupler of claim I wherein the means for controlling includesmeans for moving the radiator relative to the loop, said means formoving including a dielectric support member separating the radiatorfrom a wall of the cavity so that no electrical connections are made tothe radiator,

3. The coupler of claim 1 wherein the radiator comprises a metal studmounted with a longitudinal axis coplanar with the loop and at rightangles to the plane of magnetic flux lines coupling the loop and thestud, and the means for controlling comprises means for varying thelength of the stud in the section.

4. The coupler of claim 1 wherein the radiator comprises a metal discmounted for rotation about an axis at right angles to the plane of theloop, and the means for controlling comprises means for rotating saiddisc about said axis.

5. The coupler of claim 3 wherein the means for varying the lengthincludes a dielectric shaft extending into the section along the axisand carrying the stud so that no electrical connections are made to theradiator, the stud being mounted on the shaft so that the insertionlength of the stud and shaft in the section controls the coupling.

6. The coupler of claim 4 wherein the means for varying the lengthincludes a rotatable dielectric shaft extending into the section alongthe axis and carrying the disc so that no electrical connections aremade to the radiator, the disc being mounted on the shaft so that theangular position of the disc and shaft in the section controls thecoupling.

7. In combination with a coaxial line, a microwave cavity for supportingenergy at a frequency propagating in said line, a section in a portionof said cavity, said section being dimensioned so that it is a waveguidebeyond cutoff for the frequency propagating in said line, anelectromagnetic radiator for the energy, said radiator being positionedin the section so that electromagnetic energy is coupled between thecavity and the radiator, a termination for said line excited by theenergy propagating in said line, said termination being fixedlypositioned in the section to be inductively coupled with said radiator,whereby coupling is provided between the termination and the cavity viathe radiator of the energy inductively coupled between the terminationand radiator.

8. The combination of claim 7 wherein the cavity is resonant to thefrequency in the line, said cavity having a wall in which the section isformed.

9. The coupler of claim 8 further including means for controlling thecoupling between the loop and the radiator to control the couplingbetween the cavity and line.

10. The coupler of claim 9 wherein the means for controlling includesmeans for moving the radiator relative to the termination, said meansfor moving including a dielectric support member separating the radiatorfrom a wall of the cavity so that no electrical connections are made tothe radiator.

11. The combination of claim 10 wherein the radiator comprises a metalstud mounted with a longitudinal axis coplanar with a loop forming thetermination, said axis being at right angles to the plane of magneticflux lines in the cavity coupling the loop and the stud, the means forcontrolling comprising means for varying the length of the stud in thesection, the means for varying the length including a dielectric shaftextending through the wall into the section in alignment with the axis,said shaft carrying the stud so that no electrical connections are madeto the radiator.

12. The combination of claim 10 wherein the radiator comprises a metaldisc mounted for rotation about an axis at right angles to the plane ofa loop forming the termination, and the means for controlling comprisesmeans for rotating said disc about said axis.

1. A variable coupler between a coaxial line and a microwave cavity forsupporting electromagnetic energy at a frequency propagating in saidline comprising a section connected to and opening into said cavity,said section being dimensioned so that it is a waveguide beyond cutofffor the frequency propagating in said line, a fixedly mounted planarloop in said section connected to be in energy exchange relationshipwith the energy propagating in said line and rigidly positioned so thatits plane is substantially at right angles to a microwave magnetic fieldcoupled into the cavity, an electromagnetic radiator for the energy,said radiator being positioned in the section, said radiator being onlyinductively coupled with said loop, said radiator being positioned inthe section in electromagnetic energy exchange relationship with thecavity so that there is coupling between the cavity and the loop via theradiator of the energy inductively coupled between the loop andradiator, and means for controlling the coupling between the loop andthe radiator to control the cOupling between the cavity and line.
 2. Thecoupler of claim 1 wherein the means for controlling includes means formoving the radiator relative to the loop, said means for movingincluding a dielectric support member separating the radiator from awall of the cavity so that no electrical connections are made to theradiator.
 3. The coupler of claim 1 wherein the radiator comprises ametal stud mounted with a longitudinal axis coplanar with the loop andat right angles to the plane of magnetic flux lines coupling the loopand the stud, and the means for controlling comprises means for varyingthe length of the stud in the section.
 4. The coupler of claim 1 whereinthe radiator comprises a metal disc mounted for rotation about an axisat right angles to the plane of the loop, and the means for controllingcomprises means for rotating said disc about said axis.
 5. The couplerof claim 3 wherein the means for varying the length includes adielectric shaft extending into the section along the axis and carryingthe stud so that no electrical connections are made to the radiator, thestud being mounted on the shaft so that the insertion length of the studand shaft in the section controls the coupling.
 6. The coupler of claim4 wherein the means for varying the length includes a rotatabledielectric shaft extending into the section along the axis and carryingthe disc so that no electrical connections are made to the radiator, thedisc being mounted on the shaft so that the angular position of the discand shaft in the section controls the coupling.
 7. In combination with acoaxial line, a microwave cavity for supporting energy at a frequencypropagating in said line, a section in a portion of said cavity, saidsection being dimensioned so that it is a waveguide beyond cutoff forthe frequency propagating in said line, an electromagnetic radiator forthe energy, said radiator being positioned in the section so thatelectromagnetic energy is coupled between the cavity and the radiator, atermination for said line excited by the energy propagating in saidline, said termination being fixedly positioned in the section to beinductively coupled with said radiator, whereby coupling is providedbetween the termination and the cavity via the radiator of the energyinductively coupled between the termination and radiator.
 8. Thecombination of claim 7 wherein the cavity is resonant to the frequencyin the line, said cavity having a wall in which the section is formed.9. The coupler of claim 8 further including means for controlling thecoupling between the loop and the radiator to control the couplingbetween the cavity and line.
 10. The coupler of claim 9 wherein themeans for controlling includes means for moving the radiator relative tothe termination, said means for moving including a dielectric supportmember separating the radiator from a wall of the cavity so that noelectrical connections are made to the radiator.
 11. The combination ofclaim 10 wherein the radiator comprises a metal stud mounted with alongitudinal axis coplanar with a loop forming the termination, saidaxis being at right angles to the plane of magnetic flux lines in thecavity coupling the loop and the stud, the means for controllingcomprising means for varying the length of the stud in the section, themeans for varying the length including a dielectric shaft extendingthrough the wall into the section in alignment with the axis, said shaftcarrying the stud so that no electrical connections are made to theradiator.
 12. The combination of claim 10 wherein the radiator comprisesa metal disc mounted for rotation about an axis at right angles to theplane of a loop forming the termination, and the means for controllingcomprises means for rotating said disc about said axis.